A contrast sensitivity test is a specialized eye examination that measures how well a person can distinguish between different shades of light and dark. It evaluates the ability to perceive subtle variations in brightness, which plays a role in navigating everyday environments. This test offers a more comprehensive understanding of visual function than a standard eye chart test.
Defining Contrast Sensitivity
Contrast sensitivity refers to the eye’s capacity to discern objects that blend into their backgrounds, such as a gray car on a foggy day or the subtle contours of a person’s face. This differs from visual acuity, which measures the sharpness of vision at a distance, often using high-contrast black letters on a white Snellen chart. While visual acuity determines how clearly one can see fine details, contrast sensitivity evaluates the ability to detect subtle differences in shading and patterns. For instance, a person might achieve 20/20 visual acuity yet still struggle with poor contrast sensitivity, leading to difficulties in low light, hazy conditions, or when distinguishing unmarked steps.
The Testing Procedure
One common method involves the Pelli-Robson chart, which displays rows of capital letters. Unlike a standard visual acuity chart where letters get smaller, on the Pelli-Robson chart, the letters remain the same size but become progressively fainter as one reads down the lines. Patients identify these letters, often arranged in groups of three, until they can no longer distinguish them from the background. The test is performed at a distance of 1 meter, and the chart is uniformly illuminated.
Another assessment uses sine-wave grating charts, such as Vistech or FACT charts. These charts feature parallel light and dark bars that vary in both contrast and spatial frequency, referring to the thickness of the bars. Patients identify the orientation of these fuzzy bars—vertical, angled right, or angled left—until they can no longer perceive the pattern. This method evaluates how well the eye perceives contrast across a range of object sizes, with results indicating the lowest contrast level detectable for various spatial frequencies.
Medical Conditions Associated with Poor Results
Reduced contrast sensitivity can indicate the presence of various underlying medical conditions affecting the eyes. Cataracts, for example, cloud the eye’s natural lens, which scatters light and reduces the ability to perceive subtle shades, particularly at intermediate and high spatial frequencies. Glaucoma, a condition damaging the optic nerve, impairs the brain’s ability to detect differences in luminance, making it harder to distinguish objects from their backgrounds, especially at low spatial frequencies.
Diabetic retinopathy, a complication of diabetes, can lead to a uniform loss in contrast sensitivity even before noticeable changes in visual acuity or visible signs of retinopathy appear. Macular degeneration, which affects the central part of the retina, often results in a significant loss of contrast sensitivity, particularly in the lower spatial frequency range, impacting the ability to discern fine details and colors. Optic neuritis, an inflammation of the optic nerve often associated with conditions like multiple sclerosis, causes a significant reduction in contrast sensitivity across all spatial frequencies, even when visual acuity has seemingly recovered.